1. Field of the Invention
The present invention pertains generally to apparatus for controlling the voltage applied to a load for purposes of conserving power and more specifically, to an improved transformer apparatus for advantageously lowering the voltage applied to a load such as a bank of HID fluorescent lights after initial turn on of such lights.
2. Prior Art
The present invention constitutes an improvement over prior art U.S. Pat. No. 4,431,948, issued Feb. 14, 1984. That patent discloses a selective reduction of voltage applied to a load, such as a lighting load for purposes of reducing power consumption by utilizing an apparatus in which an autotransformer is connected is series relationship with a switching device, such as a relay. A portion of the autotransformer winding is interposed between the input and the output and an additional portion of the transformer winding is interposed between the output and the common or neutral terminal of an alternating current power system. The winding interposed between the input and the output is referred to therein as the series winding of the autotransformer and the portion of the winding interposed between the output and the common or neutral terminal is referred to therein as the common winding of the autotransformer. A relay or equivalent switching device is connected in series with the common winding so that when the relay or equivalent switching device is in its open configuration, no current can flow in the common winding and the output voltage is substantially equivalent to the input voltage, less any nominal voltage drop across the relatively low impedance of the series winding. On the other hand, when the relay or equivalent switching device is closed, the current is permitted to flow in the common winding and the autotransformer performs its normal function with the output voltage reduced relative to the input voltage in proportion to the ratio of impedance of the two windings of the autotransformer. A significant advantage of that prior art patent was the disclosure of a configuration in which it was unnecessary to break AC common and also by providing a relay or an equivalent switching device which was not in the direct path of the load current on either side of the transformer, thus allowing the use of a relay or equivalent switching device which need only handle a fraction of the actual load current. As a result, it was possible to use a relay or switching device which is smaller and far less expensive than devices of the prior art designed to carry out similar functions for reducing voltage to a load, such as a bank of lights.
However, subsequent to the issuance of the aforementioned patent, it was discovered that the disclosed circuit also suffers a significant disadvantage. This disadvantage occurred during maximum voltage application when the relay or equivalent switching device was in its open configuration, thus disconnecting the common winding of the autotransformer from the series winding of the autotransformer. More specifically, it was found that in the full voltage mode, the current through the series winding produced a significant magnetic field which induced magnetic flux saturation in the metal portion of the common winding which caused two problems. One such problem was that the magnetic field caused the common winding to produce a voltage which caused the common winding, even though disconnected electrically from the series winding, to induce harmonic distortion in the series winding which effectively reduced the amplitude of the voltage available from the series winding during the full voltage configuration. The reduction in voltage was significant, in fact significant enough to, in some cases, make it difficult or impossible to turn fluorescent or other arc type light systems "ON" during the high voltage start-up mode. Another problem associated with the magnetic flux saturation of the common winding metal structure was that it heated the metal structure of the autotransformer so significantly that it tended to destroy the insulation in the coil, which could result in catastrophic effects.
Thus, although the advantages described in the aforementioned prior art patent are significant, the then unforeseen disadvantages make it difficult if not impossible to exploit that novel power consumption reduction configuration by making it impractical to actually implement.
There is therefore a need to provide an improved apparatus for control of low power consumption which permits the realization of the advantages of the aforementioned disclosure, namely a significant reduction in the current carrying capacity required for the relay or other equivalent switching device, but which avoids the harmonic distortion and inadvertent heating affects caused by the high magnetic fields generated in the common winding during the full voltage mode of that circuit.